Wired and wireless networks, such as mobile telecommunication networks, local area networks, positioning systems, broadcasting systems, sensor networks etc., have become essential for modern daily life. In a network, when a transmitter needs to send digital information to a receiver, the transmitter encodes the digital information to a digital base-band (BB) signal, converts the digital base-band signal to an analog BB signal, modulates (up-converts) the analog BB signal to a radio-frequency (RF) signal by mixing the analog base-band signal with a local oscillation (LO) signal generated by the transmitter, and amplify the RF signal, such that the RF signal can be transmitted via network medium (e.g., air for wireless network). When the receiver receives the RF signal, it amplifies the received RF signal, demodulates (down-converts) the RF signal to a demodulated signal by mixing the RF signal with a local oscillation signal generated by the receiver, such that the digital information can be retrieved from the demodulated signal.
For signal receiving, there are several kinds of receiver architectures. For example, a receiver of a heterodyne architecture down-converts RF band to an intermediate frequency (IF) band which locates between the base-band and the RF band. On the other hand, a receiver of a direct-conversion (or zero-IF) architecture directly down-converts RF band to base-band. Comparing to the direct-conversion architecture, the heterodyne architecture demonstrates advantages, and thus provides an attractive solution for signal receiving. In addition, a receiver of a heterodyne architecture can adopt a low-IF receiving scheme, wherein a lower bound of the IF band is arranged to be close to zero frequency (DC, direct current); such arrangement proves beneficial for signal receiving performance and circuitry implementation, etc.
While heterodyne architecture is advantageous, it introduces image issue during down-conversion, wherein fluctuation (noise, interferer and/or unwanted signal) in an image band is translated to the IF band which contains desired signal. The image band is mirrored from the IF band about zero frequency; that is, the image band and the IF band locate symmetrical around zero frequency. To reduce affection of the image issue, image rejection is demanded.